Ultra-Low Frequency and Very Low Frequency (VLF) communication systems (0.3-3 kHz and 3 kHz-50 kHz, respectively) have been used for many decades for a broad range of applications. These long-wavelength bands have applications not possible at higher frequencies. This is due to a few advantageous characteristics. While coupling to the earth-ionosphere waveguide, VLF signals have path attenuation less than 3 dB/1000 km. In addition, because the skin effect in materials is inversely proportional to frequency, VLF signals can penetrate 10's of meters into seawater or the earth, while higher frequency signals quickly are attenuated. For example, underwater communication with submarines is presently accomplished through large VLF transmitters located at many locations around the world.
Efficient VLF transmitters have traditionally necessitated radiating elements at the scale of the wavelength: several kilometers. This is because the radiation resistance, Rrad, of an electric dipole which scales as (L/λ0)2 where L is the electrical length of the antenna and λ0 is the free space wavelength of the transmitting frequency. The radiation efficiency scales as Rrad/Rtotal where Rtotal is the total resistance of the antenna system including effects such as copper losses. Therefore, as the physical size of the antenna decreases, unless antenna losses are proportionally reduced, the efficiency dramatically reduces. This effect is exacerbated in the case of magnetic dipoles as the radiation resistance scales as (L/λ0)4.
Traditional metallic antennas much shorter than the radiating wavelength require large charge separation (dipole moments) and have huge input impedances, impractical for efficient and compact operation. To generate the large currents necessary to overcome their fundamentally low radiation efficiency, very high input voltages and impedance-matching networks are typically required.
What is needed is an antenna that is based upon the mechanical manipulation of charges, particularly in the Very Low Frequency (VLF, 3-30 kHz) band, to enable transmitters with a size and power consumption compatible with man-portable applications capable of closing communication links at distances greater than 100 km.